Aircraft: DHC8
Phase of flight: Parked
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy

Previous Station did not install the Weather Radar; only the Transeiver and we should [have] caught it on the install of the commercial radio avionics coupler; we were waiting on parts to arrive to fix the aircraft. The parts were scheduled to arrive 15 minutes before the flight was scheduled to depart. They were supposed to arrive earlier but the flight it was coming on kept getting delayed. [Mechanic Y]; the Mechanic who was going to replace the [avionics] coupler was a Swing-shift Mechanic and it was starting to encroach the end of his shift. I told Maintenance Control that the plane would not be ready for its flight and they should swap it with our Routine Overnight (RON) [aircraft] that we were scheduled to do a Line Check on. The [Maintenance] crew was already wrapping up the Line check and that plane would be ready sooner than this one. The part finally arrived along with some other robbed [cannibalized] parts that needed to be installed as well. I went out to the plane to help and see where the Mechanic was at. He had installed the Timer/Monitoring Unit (TMU) and was working on the avionics coupler. I closed up the wardrobe while he finished installing the coupler. We were hoping to solve two discrepancies with the coupler. A Weather Radar fail and an FMS 'Configuration Module Fail' [message]. After the installation we turned on the FMS and the 'Configuration Module Fail' message returned. We decided not to test the Weather Radar then because the FMS 'fail' returned. With the clock ticking we decided to return to the Shop to do more research into the problem; get a turnover and Mechanic Y could sign-off what he had done. He signed off the install and wrote up the Operational (Ops) Check of the Weather Radar. I told him I would take care of the Operational test of the Weather Radar System. This is where the error in communication and technical publication happens. When I go to do the Operations Test; I only take the Operational Test. This task says nothing about needing to remove the radome. When the Mechanic turned it over to me he left both tasks. I had informed him that I had just recently worked this Operations Check and was well aware of it so he probably took from that that I knew I needed to remove the Radome. In the end the Operational Test check was good; Weather Transceiver (WX) fail message did not return because the Operational Test can be passed with only the Transceiver installed and I signed it off. I have two other mechanics [that] were with me for the entire Operations Check. [Contributors were] the previous Mechanic at another Station not installing the whole part assembly. Us not catching it when we replaced the commercial radio [Avionics] coupler because we didn't remove the radome and the communication breakdown between where Swing shift left and Grave shift picked up. [Recommend] putting the removal of the radome in the Operational Test of the Weather Radar [Unit]. A Line Aircraft Maintenance Technician (AMT) describes the chain of events that led to a repetitive Weather Radar and FMS 'Configuration Module Fail' message on a DHC-8-400 aircraft.